In recent years, in order to increase the recording density of a memory magnetic disk, the surfacing thickness of the magnetic head has become extremely small such as 10 nm or less. For the production process of the magnetic disk substrate, a surface polishing process is essential and the surface polishing is performed with a polishing agent containing colloidal silica, or the like.
For the polishing agent, there is required, besides having advantageous surface smoothing properties (for example, surface roughness [Ra] and waviness [wa]), not causing a surface defect such as scratching and pitting.
An alkali silicate aqueous solution becoming a raw material for a silica sol used as a raw material for the polishing agent has conventionally been purified by a method including: adding a filtering assistant such as a diatomaceous earth to a crude alkali silicate aqueous solution immediately after being prepared by heating-dissolving a raw material cullet in the alkali silicate aqueous solution; and filtering the alkali silicate aqueous solution. As a method for obtaining an alkali silicate aqueous solution in which a particle having a size of 1 nm or more does not substantially exist, there is disclosed a method including: adjusting the viscosity of an alkali silicate aqueous solution to 1 mPa·s to 50 mPa·s beforehand; and passing the alkali silicate aqueous solution through a ultrafiltration membrane having a cutoff molecular weight of 15,000 or less (Patent Document 1).
Also in the semiconductor field, reduction in the size of wiring has been progressed in association with high integration of the circuit and speeding up of the operating frequency. In the production process of a semiconductor device also, further smoothing of the pattern forming face is desired.
In the planarizing process of the magnetic disk substrate or the semiconductor substrate, following a polishing process with a polishing agent containing colloidal silica, the removal of colloidal silica which is an abrasive grain and fine particles is performed by washing.
For washing, an aqueous solution of an acidic or alkaline chemical is used. Examples of the acidic chemical include: a fluorine ion-containing compound such as hydrofluoric acid, ammonium fluoride, ammonium hydrogen difluoride, and fluoroboric acid; sulfuric acid; nitric acid; hydrochloric acid; acetic acid; citric acid; malic acid; oxalic acid; and perchloric acid. Examples of the alkaline chemical include sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonia, and amines. In addition, to such an acidic or alkaline chemical, a surfactant such as sodium alkylbenzenesulfonate, polyoxyethylene alkyl ether sulfate, and dioctylsulfosuccinate or a chelating agent such as sodium tripolyphosphate, sodium pyrophosphate, zeolite, and sodium ethylenediaminetetraacetate may be added as a component.
The colloidal silica used for the above polishing agent is in a spherical shape or a substantially spherical shape, so that such a colloidal silica can be removed by a conventional washing method. However, recently, it has become apparent that there exists a plate-like fine particle which cannot be easily removed by a conventional washing method.